Primary cultures of peripheral blood leukocytes from a series of unrelated human donors were stimulated in vitro against TNP-modified autologous leukocytes. Weak primary proliferative responses were detected using 3H-thymidine pulsing of the responding cells. The cultures were restimulated with TNP-modified or unmodified autologous or allogeneic leukocytes. Strong proliferative responses were detected preferentially in cultures restimulated with TNP-modified autologous cells, or in modified allogeneic cells sharing an HL-A A locus specificity with the modified primary stimulating cells. BIBLIOGRAPHIC REFERENCE: Shearer, G.M., Schmitt-Verhulst, A.-M., Shaw, S., Pettinelli, C., Henkart, P., and Rehn, T.G.: The role of MHC genes in T-cell mediated responses to syngeneic modified cells. Proceedings of Symposium on Molecular and Cellular Biology, Park City, Utah. March 21-25, 1977.